In recent years, a wavelength of exposure light emitted from a light source is reduced along with miniaturization and high integration of semiconductor integrated circuits. As a next-generation light source for semiconductor exposure, a light source device (hereinafter occasionally referred to as an “EUV light source device”) for emitting extreme ultraviolet light (hereinafter occasionally referred to as “EUV light”) particularly having a wavelength of 13.5 nm is developed.
The EUV light source device may use one of known methods to generate the EUV light. One of such methods includes heating and exciting an extreme ultraviolet light emission seed (EUV emission seed) to generate high-temperature plasma, and taking out the EUV light from the high-temperature plasma.
The EUV light source devices employing such method are classified into an LPP (Laser Produced Plasma) type and a DPP (Discharge Produced Plasma) type, depending upon the employed manner of generating the high-temperature plasma.
A DPP type EUV light source device applies a high voltage across a pair of electrodes, which are supplied with a discharge gas containing an extreme ultraviolet light emission seed, to generate high-density and high-temperature plasma upon electric discharge, and uses extreme ultraviolet light emitted from the high-density and high-temperature plasma. For use with the DPP type EUV light source device, a method has been proposed that includes supplying a surface of one of the electrodes, which are used to generate the electric discharge, with a raw material such as Sn (tin) or Li (lithium), and irradiating the raw material with an energy beam such as a laser beam for evaporation thereof such that high-temperature plasma is generated upon the electric discharge. Such method is sometimes referred to as an LDP (Laser Assisted Discharge Produced Plasma) method or an LDP type.
Technology of an LDP-type EUV light source device is disclosed in, for example, Patent Literature Document 1 (Japanese Patent No. 4623192). This technology of the LDP-type EUV light source device includes a first laser source that irradiates a raw material, which is delivered onto an electrode, with a first laser beam, to vaporize the raw material thereby triggering the discharge between two electrodes, and also includes a second laser source that, after irradiating the raw material with the first laser beam and before triggering the discharge between the two discharge electrodes, irradiates the raw material on the discharge electrode in that region which is irradiated with the first laser beam, with a second laser beam to further vaporize the raw material. This technology of the LDP-type EUV light source device emits the second laser beam to supply the discharge area with a gas of high-density plasma raw material, thereby obtaining an efficient EUV radiation.